T cells are important effector cells in natural antiviral and anticancer immunity, and it is the ultimate goal of our studies to reveal the cellular and molecular requirements for T cell differentiation and effector functions. We continued exploration of a novel mechanism of T cell regulation that is based on the assumption of important roles of extracellular purines (ATP and adenosine) and purinergic receptors. Signaling through these receptors is propagated by phosphorylation/ dephosphorylation processes and results in modulation of TCR-driven effector functions of T-cells. It has been shown that expression of Gprotein-coupled purinergic receptors in activated thymocytes follows the pattern of an immediate early gene response where newly expressed cell surface purinergic receptors may provide regulatory feed-back signaling. The demonstration of TCR-antagonizing effects of extra cellular adenosine (extAdo) led us to propose an extAdo-based mechanism of an inherited disease, severe combined immunodeficiency (SCID), in adenosine deaminase (ADA)-deficient patients. In contrast to the current explanation of ADA SCID by the general lymphotoxicity of intracellular adenosine, we suggested the possibility of adenosine receptor-induced thymocyte and Tcell depletion. It has been shown that low concentrations of adenosine strongly antagonized antigenic peptide or anti-TCR mAb-induced upregulation of activation markers and apoptosis. ExtAdo has especially strong effects on TCR-activated thymocytes with low or no adenosine deaminase (ADA) activity. The overall effect of extAdo was the "rescue" (increased short-term survival) of TCR-activated thymocytes in an in vitro assay that models negative selection of thymocytes. The A2a purinergic receptors are shown to be the predominant receptors for extAdo, which signal to increase levels of cAMP and to activate cAMP-dependent protein kinase in Tcells. According to our model, the extAdo may cause T cell depletion by A2a receptor-mediated abrogation of the TCR signaling required for positive selection, thereby allowing thymocytes to die from "neglect." Similarly, the autoimmunity observed in ADA SCID patients is explained by the ability of extAdo to antagonize TCR-mediated signaling needed for negative selection, thereby allowing the survival of autoimmune T cells. We also demonstrated strong inhibitory effects of extAdo on TCR-triggered effector functions of T cells, thereby suggesting that in tissues with a high concentration of adenosine (e.g., hypoxic conditions of large solid tumors), the immune response could be inhibited. These observations point to A2a receptors as a novel molecular target for immunomodulation.